The control of corrosion of metal structures below ground level has long been and continues to be a problem which plagues utility service companies in all geographical areas. The problem is particularly acute in connection with underground pipe lines which frequently are buried in relatively inaccessible locations both urban areas as well as rural geographical sties. In the case of gas transmission pipe lines for example, it is absolutely essential that the pipe be protected to mitigate corrosion, and costs associated with such protection have been significant in the past and are progressively increasing as the length of time the pipe has been buried increases. If corrosion "hot spots" develop along the pipe line, remedial action adds additional cost to the operation, especially if the only alternative is to replace the pipe which has become corroded.
The two most common methods of protecting pipe are coatings and cathodic protection. Usually the two are used in combination. Pipe lines are normally coated to insulate the pipe from the corrosive action of the soil. However, coatings invariably have flaws (referred to as "holidays") which can act as sites for corrosion. Therefore, cathodic protection is applied to the pipe to prevent the flaws or holidays from becoming active corrosion sites. Cathodic protection involves the use of a small electric current to create a film of hydrogen gas at the holidays to insulate them from the soil.
Since a coated pipe line is usually more than 99% covered, the amount of current needed for cathodic protection of the remaining 1% or less is relatively small. However, when consideration is taken into account of the many miles of coated pipe which are under cathodic protection, the amount of power required to maintain the added protection has a significant impact on overall operating costs. The problem becomes exacerbated when hot spots develop on the pipe which require additional current to maintain the necessary pipe-to-soil potential for cathodic protection.
Another type of protection for pipes takes the form of sacrificial electrodes which are electrically connected to the pipe at intervals along the length thereof. The most usual sacrificial electrode in this respect is made up of magnesium in a suitable medium. Here again, holidays in a pipe result in an increased depletion of the sacrificial metal and the requirement of replacing the same at more frequent intervals than would otherwise be the case if the surface integrity of the pipe coating had not been disrupted.